A planar waveguide laser has a structure in which a thin, plate-like laser medium that extends in a laser light traveling direction is sandwiched from its top and bottom surfaces by a clad that has a lower refractive index than that of the laser medium. The structure causes the laser medium to function also as a waveguide. Because of its small waveguide thickness and high excitation density, the planar waveguide laser can attain a high gain even when a laser medium having a small cross section area for stimulated emission is adopted. This realizes an oscillation operation with a high degree of efficiency. Furthermore, by widening the waveguide in its width direction, scaling of the output can be realized while the excitation density is maintained at a predetermined value. However, the gain is high and oscillations easily occur in multiple waveguide modes. Thus, challenges are to oscillate laser light with linear polarization that is required at the time of wavelength conversion and to oscillate laser light restricted to a desired mode. Further challenges are to output laser light with high efficiency by reducing amplification of unnecessary light (parasitic amplification) and parasitic oscillation, which occurs by the trapping of light in the waveguide because of the total reflection on the external surfaces and end surfaces of the clad.
Therefore, planar waveguide laser devices that realize laser oscillation in a desired mode have been conventionally suggested (refer to Non-patent Document 1, for example). This planar waveguide laser device has a structure in which a waveguide core is formed by incorporating Nd:YAG (yttrium aluminum garnet: Y3Al5O12) as a core and attaching a non-doped YAG to the top and bottom surfaces of the core, and this waveguide core is further sandwiched by a sapphire crystal that serves as the clad layers from the top and bottom surfaces (i.e., the non-doped YAG surfaces). In such a waveguide structure, oscillation can occur in more than one waveguide mode. However, it is possible to cause oscillation only in the waveguide basic mode by making the core to have a gain at only around its center so as to cause a gain difference between the waveguide basic mode and the waveguide higher mode. Also, the basic mode can be selectively oscillated as the oscillation mode in the width direction of the waveguide by conforming the incident form of the excitation light to the form of the basic mode.    Non-patent Document 1: Jacob I. Mackenzie, Cheng Li, and David P. Shepherd, “Multi-Watt High Efficiency, Diffraction-limited Nd:YAG Planar Waveguide Laser”, IEEE Journal of Quantum Electronics, Vol. 39 (2003), p. 495